1. Field of the Invention
This invention relates generally to aluminum nitride (AlN) and more particularly, to epitaxial cubic (zinc-blende) AlN films that may have a thickness on the order of 1000 xc3x85 or greater and a method of making same by plasma source molecular beam epitaxy (PSMBE).
2. Description of the Related Art
The Group III-V nitride semiconductors (GaN, AlN, and InN) are of great interest for their potential as optoelectronic materials. These materials have an equilibrium crystal structure which is wurtzite, or hexagonal. The bandgaps of the wurtzite nitride semiconductors are all direct and their alloys have a continuous range of direct bandgaps values ranging from 1.9 eV for InN to 4.0 eV for GaN to 6.2 eV for AlN. As optical materials, these semiconductors are active from the orange into the ultraviolet.
Formation of nitride semiconductors for device applications requires, among other things, achieving the correct stoichiometry, inducing the correct energy to form a highly crystalline matrix, maintaining high purity, and matching the lattice parameters of the semiconductor and the substrate. Much effort was expended in the 1960""s and 1970""s to grow and characterize Group III-V nitride semiconductors. However, the effort was ineffectual to achieve high-quality material. Recently, there has been renewed effort to create higher quality Group III-V nitride semiconductors. However, GaN, AlN, and InN produced by conventional methods have high n-type background carrier concentrations resulting from native defects commonly thought to be nitrogen vacancies. Nitrogen vacancies affect the electrical and optical properties of the film. Oxygen contamination is also a major problem. Thin layers of AlN have been prepared by magnetron sputtering, chemical vapor deposition, ion beam sputtering, and ion beam assisted deposition. However, these methods operate at elevated temperatures and generally do not result in epitaxial growth (i.e., growth oriented in one direction). Moreover, while these techniques have been successful in producing polycrystalline AlN films, they have not been successful in producing electronic-grade single crystal films.
AlN, in particular, is a promising material for high-power, high-temperature optoelectronic devices since it has very high chemical and thermal stability, good thermal conductivity, and fast Rayleigh velocity. AlN crystallizes, under normal conditions, into the thermodynamically stable hexagonal wurtzite structure. However, the metastable cubic zinc-blende structure is expected to be easier to dope and to have decreased phonon scattering, and therefore, to have higher ballistic electron velocities, thermal conductivity, and acoustic velocities due to its higher symmetry. These properties give rise to many exciting potential device applications.
There have been several reports of AlN having the metastable cubic zinc-blende structure. These reports, however, lack detail on the physical, electrical, and optical properties of cubic zinc-blende AlN because the films were too thin for such studies, and certainly too thin to be useful for optoelectronic devices which require thicknesses on the order of at least 2000 xc3x85, and preferably 4000 xc3x85 to 8000 xc3x85. The lattice constant of zinc-blende AlN was calculated theoretically to be 4.38 xc3x85 using data from the elastic constants of wurtzite AlN. This value was later confirmed experimentally on a 12 nm thick film of zinc-blende AlN grown pseudomorphically on cubic TiN sandwiched between a tetragonal Al3Ti overlayer. To date, however, there have been no reports of successful fabrication of thick, device-quality films of zinc-blende AlN. The known AlN films have been mixed hexagonal and possibly cubic (which could be the rock salt structure).
It is, therefore, an object of the invention to prepare zinc-blende AlN of sufficient quality and thickness to characterize it for its mechanical, optical, and electrical properties and to be useful for device fabrication.
It is also an object of the invention to prepare device quality, single crystal, epitaxial films of cubic zinc-blende AlN.
It is a further object of the invention to produce a semiconductor devices that include an epitaxial film(s) of single crystal zinc-blende AlN.
It is an additional object of this invention to provide a method of making an epitaxial film of zinc-blende AlN.
The foregoing and other objects, features and advantages are achieved by this invention which is, in a first device embodiment, a film of device quality, single crystal cubic zinc-blende AlN. In other embodiments, the zinc-blende AlN film is deposited on a substrate, and preferably on a substrate having cubic symmetry on its surface, such as a silicon (100) wafer (Si(100)). In a particularly preferred embodiment, there is a buffer, or an interfacial, layer of cubic 3Cxe2x80x94SiC between the epitaxial film of zinc-blende AlN and the substrate which may be a Si(100) wafer.
In a specific illustrative embodiment, a semiconductor device comprises a Si(100)-oriented substrate; an interfacial layer of 3Cxe2x80x94Si(C) on the Si(100) substrate; and a film of single crystal zinc-blende AlN having a thickness of at least 800 xc3x85, and preferably in the range of 1000 xc3x85 to 2000 xc3x85, which is epitaxial with respect to the Si(100) substrate. The epitaxial relationship between film and substrate is (100)AlN∥(100)Si and [101]AlN∥[101]Si. The interfacial layer may have a thickness ranging from several atomic layers (e.g., about 25-30 nm) and up.
In accordance with the principles of the invention, the metastable zinc-blende form of AlN is deposited on the substrate by a plasma beam of aluminum ions and activated nitrogen ion species produced in a molecular beam epitaxy system by applying a pulsed d.c. power to a hollow cathode source. In this manner, films having a thickness of at least 800 xc3x85 were produced. Thickness, of course, is a function of deposition time, and films ranging from 10 xc3x85 to several microns, are possible by the method of the present invention. The lattice parameter of the as-deposited films was calculated to be approximately 4.373 xc3x85 which corresponds closely to the theoretical calculation (4.38 xc3x85) for cubic zinc-blende AlN.
The zinc-blende AlN epilayer films of the present invention have a wide bandgap (experimentally determined to be about 5.34 eV); thermal stability (up to about 800xc2x0 C.), and extraordinary piezoelectric properties. In addition to the foregoing, the films have been of sufficient quality to enable experimental confirmation that zinc-blende AlN is an indirect semiconductor. When characterized in situ by Reflection High Energy Electron Diffraction (RHEED), the films show four-fold symmetry rather than the six-fold symmetry which is typical for hexagonal AlN in (0001) orientation. Furthermore, the RHEED patterns appear to be very similar to those for the Si(001) substrates, except for different streak spacings. X-ray diffraction (XRD) revealed broad peaks at diffraction angle (2xcex8) values of approximately 41xc2x0 and 89.8xc2x0. These peaks match the (002) and (004) peaks of zinc-blende AlN with a lattice parameter of 4.38 xc3x85. Transmission electron microscopy (TEM) confirmed that the AlN produced by the method of the present invention is cubic, single crystal and epitaxial with respect to the Si(100) substrate.
In accordance with the invention, the growth surface of the substrate should preferably have a cubic structure to act as a template for cubic (zinc-blende) growth. Specific illustrative examples include, but are not limited to Si(100) and magnesium oxide (MgO (100)). Of course, the substrate may comprise one or more layers. Preferably, the growth surface of the substrate should have an good lattice match with AlN. A good lattice match is defined as being within about 1%. However, as is known in the art, substrates with seemingly poor lattice matches (e.g, Si(100) has a 19% mismatch), may be used since epitaxially deposited layers have mechanisms to compensate for the mismatch, such as by forming defects.
3Cxe2x80x94SiC is an example of a substrate that has less than 1% lattice mismatch with AlN. In a particularly preferred embodiment, there is a buffer layer, or an interfacial layer, of cubic 3Cxe2x80x94SiC between the epitaxial film of zinc-blende AlN and a Si(100) wafer. The 3Cxe2x80x94SiC layer may have a thickness ranging from several atomic layers (e.g., about 25-30 nm) and up. A silicon substrate with a 3Cxe2x80x94SiC layer can be purchased from a commercial source or made in the laboratory. The 3Cxe2x80x94SiC layer can be deposited on a silicon substrate by any number of known, high temperature processes, such as chemical vapor deposition or enhanced MBE. As will be described more completely hereinbelow, 3Cxe2x80x94SiC can be made in situ on a silicon substrate by the methods in accordance with the invention herein.
In a specific illustrative embodiment of one of the many device applications contemplated by the invention, an interdigitated transducer surface acoustic wave (SAW) device comprises a Si(100) substrate, an epitaxial film of single crystal, zinc-blende film of AlN deposited on the substrate, and two interdigitated electrodes deposited on the epitaxial AlN film by standard photolithographic techniques.
In a method embodiment of the present invention, a single crystal, epitaxial AlN film having a zinc-blende cubic structure is formed by exposing a heated substrate to a low energy flux of target atoms in an ultrahigh vacuum PSMBE system. The PSMBE system uses a plasma deposition source which is a magnetically-enhanced, generally cylindrical hollow chamber comprising a cathode. The chamber is lined with the target material which, in the present case, is MBE-grade aluminum. The target material is milled so that its thickness is greater at the upper, or exit, end of the chamber than the thickness at the lower end. In a preferred embodiment of the invention, there is about a 3xc2x0 internal taper in the chamber. Plasma is formed in the chamber by the application of d.c. or r.f. power. The application of a pulsed d.c. power produces an epitaxial layer of metastable zinc-blende AlN. The magnetic field and the taper of the interior of the cathode cooperate to confine the plasma to the cathode. The low energy flux of target atoms is extracted from the exit end of the chamber either by the action of an impeller rotatably mounted in the cathode source or by an acceleration bias applied to the substrate.
Secondary electrons are confined to the hollow cathode source and do not interact with the substrate which is mounted distant from the exit end of the chamber by, in a specific preferred embodiment, approximately 25 cm. Due to the extreme anisotrophy of the kinetically ejected atoms perpendicular to the source wall and the tapered geometry of the hollow cathode chamber, virtually no high-energy atoms or ions are directed to the substrate surface. Instead, the atoms go through multiple collisions and thermalize. Since ions are extracted from the source via an impeller or acceleration bias, the energy distribution is controlled. Mass spectrometry energy analysis of the non-accelerated ions ejected from the r.f. powered source indicates a gaussian energy distribution about the approximate 1 eV range.
The PSMBE system of the present invention, in contrast to planar sputtering systems, allows enough adatom energy to create AlN crystals (or other semiconductor nitrides) while minimizing damage to the underlying crystal substrate. When pulsed d.c. power is used instead of r.f.power, the prolonged high potential state of the pulse combined with the delay time between pulses results in more energetic plasma with high plasma and Al/N kinetic energies. The increased intensity of the plasma gives more dissociated and hence, more energetic, aluminum and nitrogen species for deposition on the substrate. This increase in the plasma energy and active energetic species assists in the formation of cubic zinc-blende AlN.
In a further method aspect of the present invention, the substrates are pre-treated to de-grease the substrate with solvents and to remove surface oxidation. The substrate is then, preferably pre-heated to a high temperature, illustratively approximately 800xc2x0 C., for one hour before deposition of the epixtaxial film.
In a specific illustrative embodiment, a Si(100) substrate is pre-treated by (i) ultrasonic cleaning in acetone for 20 minutes; (ii) blow-drying with dry nitrogen; (iii) ultrasonic cleaning in methanol for 20 minutes; and (iv) blow-drying with dry nitrogen. The substrate is then etched in an acid, illustratively 10% (by volume) HF to remove the SiO2 layer from the surface and to saturate dangling Si bonds with hydrogen atoms. The substrate is then loaded into the deposition chamber, preferably within 10 minutes of treatment.
In a particularly preferred embodiment, the pre-treated Si(100) substrate is subjected to a source of carbon to combine with the Si, during the pre-heat, to create an interfacial layer of cubic 3Cxe2x80x94SiC on the Si(100) of at least several atomic layers thickness. The cubic 3Cxe2x80x94SiC acts as a template for the deposition of cubic zinc-blended AlN. Illustratively a source of carbon can be introduced during the etching step by including trace amounts of a mixture of hydrocarbons in the etching solution. For example, the hydrocarbons may be the resins found in standard photoresist materials.
In an additional embodiment of the invention, a PSMBE system useful in preparing epitaxial films on a substrate comprises an ultrahigh vacuum growth chamber. An ultrahigh vacuum pump is connected to the growth chamber so that base pressures in the range 10xe2x88x929-10xe2x88x9210 torr can be achieved and maintained.
A substrate holder is, preferably, rotatably mounted in the interior upper end of the growth chamber. The substrate holder is heated to bring a substrate mounted thereon to growth, or processing, temperature. A substrate bias power supply is electrically connected to the substrate holder.
At least one plasma deposition source, and preferably from two to eight sources, is located distal to the substrate holder to achieve an appropriate distance, preferably in the range of 5 cm to 50 cm, from the exit end of the source to the deposition, or growth, surface of the substrate. The plasma deposition source, in a preferred embodiment, is a magnetically-enhanced hollow cylindrical cathode lined with target material which, in this specific embodiment, is aluminum. The plasma deposition source includes an inlet for high purity process gases, specifically nitrogen and argon. A source of pulsed d.c. or r.f. power is electrically connected to the plasma deposition source for creating a of plasma of high-energy aluminum, nitrogen, and argon species. An impeller is rotatably mounted in the plasma deposition source for ejecting ions from the source. The cathode source is surrounded by an anode, which may be a stainless steel enclosure, at an insulating distance from the cathode.
In a preferred embodiment of the invention, the plasma deposition source is a cathode comprising a hollow, generally cylindrical chamber, the chamber having an opening at the upper end for emitting the generated plasma and an inlet at the lower end for the process gases. The source includes magnetic field generating means. In this specific embodiment, the magnetic field generating means is an array of magnets embedded in aluminum and forming the exterior wall of the chamber. The magnetic field confines the plasma to the hollow chamber. The target material forms the interior lining of the chamber wall. The thickness of the target material lining the chamber increases from the lower end to the upper end to provide a taper that further confines the plasma to the interior of the chamber. In a specific embodiment, the lining has a 3xc2x0 taper.